|Abstract:||This paper addresses three important observations relating to cross-linguistic patterns of metathesis. First, the direction of change in metathesis can differ from language to language such that a similar sound combination can be realized in one order in one language, but in the reverse order in another language. Second, for some sound combinations, one order is favored cross-linguistically as the output for metathesis, while for others, one order occurs just as frequently as the other. Third, the acoustic/auditory cues to the identification of the sequence resulting from metathesis are frequently better, or optimized, as compared to those of the expected, yet non-occurring, order. These patterns receive a straightforward account when we consider the nature of the sounds involved, and a speaker/hearer's native language knowledge. It is argued that for metathesis to occur, two conditions must be met: first, there must be ambiguity in the signal and second, the order of sounds opposite to that occurring in the input must be attested in the language. With respect to this last point, it is shown that an individual's knowledge of his/her language, including its patterns of usage, is an effective predictor of metathesis. Support for this approach comes from the metathesis patterns themselves, as well as a large body of research in phonetics, phonology, historical linguistics and psycholinguistics. While neither the phonetic nature of the sounds involved nor knowledge of native sound sequences and their usage is sufficient to provide a fully predictive account of metathesis, this study shows that by taking into account both factors, we are able to understand why metathesis occurs, why it favors certain sound combinations, and why we obtain the output that we do. Implications of the present study for phonology, the nature of Optimality theoretic constraints in particular, are noted in the final section.